iptables has a fairly detailed manual page (man iptables),
and if you need more detail on particulars. Those of you familiar
with ipchains may simply want to look at
Differences Between iptables and ipchains; they are very
similar.

There are several different things you can do with iptables.
You start with three built-in chains INPUT, OUTPUT
and FORWARD which you can't delete. Let's look at the
operations to manage whole chains:

Create a new chain (-N).

Delete an empty chain (-X).

Change the policy for a built-in chain. (-P).

List the rules in a chain (-L).

Flush the rules out of a chain (-F).

Zero the packet and byte counters on all rules in a chain (-Z).

There are several ways to manipulate rules inside a chain:

Append a new rule to a chain (-A).

Insert a new rule at some position in a chain (-I).

Replace a rule at some position in a chain (-R).

Delete a rule at some position in a chain, or the first that matches (-D).

iptables may be a module, called (`iptable_filter.o'), which should be
automatically loaded when you first run iptables. It can
also be built into the kernel permenantly.

Before any iptables commands have been run (be careful: some
distributions will run iptables in their initialization scripts),
there will be no rules in any of the built-in chains (`INPUT',
`FORWARD' and `OUTPUT'), all the chains will have a policy of ACCEPT.
You can alter the default policy of the FORWARD chain by providing the
`forward=0' option to the iptable_filter module.

This is the bread-and-butter of packet filtering; manipulating rules.
Most commonly, you will probably use the append (-A) and delete (-D)
commands. The others (-I for insert and -R for replace) are simple
extensions of these concepts.

Each rule specifies a set of conditions the packet must meet, and what
to do if it meets them (a `target'). For example, you might want to
drop all ICMP packets coming from the IP address 127.0.0.1. So in
this case our conditions are that the protocol must be ICMP and that
the source address must be 127.0.0.1. Our target is `DROP'.

127.0.0.1 is the `loopback' interface, which you will have even if you
have no real network connection. You can use the `ping' program to
generate such packets (it simply sends an ICMP type 8 (echo request)
which all cooperative hosts should obligingly respond to with an ICMP
type 0 (echo reply) packet). This makes it useful for testing.

You can see here that the first ping succeeds (the `-c 1' tells ping
to only send a single packet).

Then we append (-A) to the `INPUT' chain, a rule specifying that for
packets from 127.0.0.1 (`-s 127.0.0.1') with protocol ICMP (`-p icmp')
we should jump to DROP (`-j DROP').

Then we test our rule, using the second ping. There will be a pause
before the program gives up waiting for a response that will never
come.

We can delete the rule in one of two ways. Firstly, since we know
that it is the only rule in the input chain, we can use a numbered
delete, as in:

# iptables -D INPUT 1
#

To delete rule number 1 in the INPUT chain.

The second way is to mirror the -A command, but replacing the -A with
-D. This is useful when you have a complex chain of rules and you
don't want to have to count them to figure out that it's rule 37 that
you want to get rid of. In this case, we would use:

# iptables -D INPUT -s 127.0.0.1 -p icmp -j DROP
#

The syntax of -D must have exactly the same options as the -A (or -I
or -R) command. If there are multiple identical rules in the same
chain, only the first will be deleted.

We have seen the use of `-p' to specify protocol, and `-s' to specify
source address, but there are other options we can use to specify
packet characteristics. What follows is an exhaustive compendium.

Specifying Source and Destination IP Addresses

Source (`-s', `--source' or `--src') and destination (`-d',
`--destination' or `--dst') IP addresses can be specified in four
ways. The most common way is to use the full name, such as
`localhost' or `www.linuxhq.com'. The second way is to specify the IP
address such as `127.0.0.1'.

The third and fourth ways allow specification of a group of IP
addresses, such as `199.95.207.0/24' or `199.95.207.0/255.255.255.0'.
These both specify any IP address from 199.95.207.0 to 199.95.207.255
inclusive; the digits after the `/' tell which parts of the IP address
are significant. `/32' or `/255.255.255.255' is the default (match
all of the IP address). To specify any IP address at all `/0' can be
used, like so:

This is rarely used, as the effect above is the same as not specifying
the `-s' option at all.

Specifying Inversion

Many flags, including the `-s' (or `--source') and `-d'
(`--destination') flags can have their arguments preceded by `!'
(pronounced `not') to match addresses NOT equal to the ones given.
For example. `-s ! localhost' matches any packet not coming
from localhost.

Specifying Protocol

The protocol can be specified with the `-p' (or `--protocol') flag.
Protocol can be a number (if you know the numeric protocol values for
IP) or a name for the special cases of `TCP', `UDP' or `ICMP'. Case
doesn't matter, so `tcp' works as well as `TCP'.

The protocol name can be prefixed by a `!', to invert it, such as `-p
! TCP' to specify packets which are not TCP.

Specifying an Interface

The `-i' (or `--in-interface') and `-o' (or `--out-interface') options
specify the name of an interface to match. An interface is
the physical device the packet came in on (`-i') or is going out on
(`-o'). You can use the ifconfig command to list the
interfaces which are `up' (i.e., working at the moment).

Packets traversing the INPUT chain don't have an output
interface, so any rule using `-o' in this chain will never match.
Similarly, packets traversing the OUTPUT chain don't have an
input interface, so any rule using `-i' in this chain will never match.

Only packets traversing the FORWARD chain have both an
input and output interface.

It is perfectly legal to specify an interface that currently does not
exist; the rule will not match anything until the interface comes up.
This is extremely useful for dial-up PPP links (usually interface
ppp0) and the like.

As a special case, an interface name ending with a `+' will match all
interfaces (whether they currently exist or not) which begin with that
string. For example, to specify a rule which matches all PPP
interfaces, the -i ppp+ option would be used.

The interface name can be preceded by a `!' with spaces around it, to
match a packet which does not match the specified
interface(s), eg -i ! ppp+.

Specifying Fragments

Sometimes a packet is too large to fit down a wire all at once. When
this happens, the packet is divided into fragments, and sent
as multiple packets. The other end reassembles these fragments to
reconstruct the whole packet.

The problem with fragments is that the initial fragment has the
complete header fields (IP + TCP, UDP and ICMP) to examine, but
subsequent packets only have a subset of the headers (IP without the
additional protocol fields). Thus looking inside subsequent fragments
for protocol headers (such as is done by the TCP, UDP and ICMP
extensions) is not possible.

If you are doing connection tracking or NAT, then all fragments will
get merged back together before they reach the packet filtering code,
so you need never worry about fragments.

Please also note that in the INPUT chain of the filter table (or any other
table hooking into the NF_IP_LOCAL_IN hook) is traversed after
defragmentation of the core IP stack.

Otherwise, it is important to understand how fragments get treated by
the filtering rules. Any filtering rule that asks for information we
don't have will not match. This means that the first fragment is
treated like any other packet. Second and further fragments won't be.
Thus a rule -p TCP --sport www (specifying a source port of
`www') will never match a fragment (other than the first fragment).
Neither will the opposite rule -p TCP --sport ! www.

However, you can specify a rule specifically for second and further
fragments, using the `-f' (or `--fragment') flag. It is also legal to
specify that a rule does not apply to second and further
fragments, by preceding the `-f' with ` ! '.

Usually it is regarded as safe to let second and further fragments
through, since filtering will effect the first fragment, and thus
prevent reassembly on the target host; however, bugs have been known
to allow crashing of machines simply by sending fragments. Your call.

Note for network-heads: malformed packets (TCP, UDP and ICMP packets
too short for the firewalling code to read the ports or ICMP code and
type) are dropped when such examinations are attempted. So are TCP
fragments starting at position 8.

As an example, the following rule will drop any fragments going to
192.168.1.1:

# iptables -A OUTPUT -f -d 192.168.1.1 -j DROP
#

Extensions to iptables: New Matches

iptables is extensible, meaning that both the
kernel and the iptables tool can be extended to provide new features.

Some of these extensions are standard, and other are more exotic.
Extensions can be made by other people and distributed separately for
niche users.

Kernel extensions normally live in the kernel module subdirectory,
such as /lib/modules/2.4.0-test10/kernel/net/ipv4/netfilter. They are demand loaded if your
kernel was compiled with CONFIG_KMOD set, so you should not need to
manually insert them.

Extensions to the iptables program are shared libraries which
usually live in /usr/local/lib/iptables/, although a distribution
would put them in /lib/iptables or /usr/lib/iptables.

Extensions come in two types: new targets, and new matches (we'll
talk about new targets a little later). Some protocols automatically
offer new tests: currently these are TCP, UDP and ICMP as shown below.

For these you will be able to specify the new tests on the command
line after the `-p' option, which will load the extension. For
explicit new tests, use the `-m' option to load the extension, after
which the extended options will be available.

To get help on an extension, use the option to load it (`-p', `-j' or
`-m') followed by `-h' or `--help', eg:

# iptables -p tcp --help
#

TCP Extensions

The TCP extensions are automatically loaded if `-p tcp' is specified.
It provides the following options (none of which match fragments).

--tcp-flags

Followed by an optional `!', then two strings
of flags, allows you to filter on specific TCP flags. The first
string of flags is the mask: a list of flags you want to examine. The
second string of flags tells which one(s) should be set. For example,

# iptables -A INPUT --protocol tcp --tcp-flags ALL SYN,ACK -j DROP

This indicates that all flags should be examined (`ALL' is synonymous
with `SYN,ACK,FIN,RST,URG,PSH'), but only SYN and ACK should be set.
There is also an argument `NONE' meaning no flags.

--syn

Optionally preceded by a `!', this is shorthand
for `--tcp-flags SYN,RST,ACK SYN'.

--source-port

followed by an optional `!', then either a
single TCP port, or a range of ports. Ports can be port names, as
listed in /etc/services, or numeric. Ranges are either two port names
separated by a `:', or (to specify greater than or equal to a given
port) a port with a `:' appended, or (to specify less than or equal to
a given port), a port preceded by a `:'.

--sport

is synonymous with `--source-port'.

--destination-port

and

--dport

are the same as
above, only they specify the destination, rather than source, port to
match.

--tcp-option

followed by an optional `!' and a number,
matches a packet with a TCP option equaling that number. A packet
which does not have a complete TCP header is dropped automatically if
an attempt is made to examine its TCP options.

An Explanation of TCP Flags

It is sometimes useful to allow TCP connections in one direction, but
not the other. For example, you might want to allow connections to an
external WWW server, but not connections from that server.

The naive approach would be to block TCP packets coming from the
server. Unfortunately, TCP connections require packets going in both
directions to work at all.

The solution is to block only the packets used to request a
connection. These packets are called SYN packets (ok,
technically they're packets with the SYN flag set, and the RST and ACK
flags cleared, but we call them SYN packets for short). By
disallowing only these packets, we can stop attempted connections in
their tracks.

The `--syn' flag is used for this: it is only valid for rules which
specify TCP as their protocol. For example, to specify TCP connection
attempts from 192.168.1.1:

-p TCP -s 192.168.1.1 --syn

This flag can be inverted by preceding it with a `!', which means
every packet other than the connection initiation.

UDP Extensions

These extensions are automatically loaded if `-p udp' is specified.
It provides the options `--source-port', `--sport',
`--destination-port' and `--dport' as detailed for TCP above.

ICMP Extensions

This extension is automatically loaded if `-p icmp' is specified. It
provides only one new option:

--icmp-type

followed by an optional `!', then either an
icmp type name (eg `host-unreachable'), or a numeric type (eg. `3'),
or a numeric type and code separated by a `/' (eg. `3/3'). A list
of available icmp type names is given using `-p icmp --help'.

Other Match Extensions

The other extensions in the netfilter package are demonstration
extensions, which (if installed) can be invoked with the `-m' option.

mac

This module must be explicitly specified with `-m mac'
or `--match mac'. It is used for matching incoming packet's source
Ethernet (MAC) address, and thus only useful for packets traversing
the PREROUTING and INPUT chains. It provides only one option:

--mac-source

followed by an optional `!', then an
ethernet address in colon-separated hexbyte notation, eg
`--mac-source 00:60:08:91:CC:B7'.

limit

This module must be explicitly specified with `-m
limit' or `--match limit'. It is used to restrict the rate of
matches, such as for suppressing log messages. It will only match a
given number of times per second (by default 3 matches per hour,
with a burst of 5). It takes two optional arguments:

--limit

followed by a number; specifies the maximum
average number of matches to allow per second. The number can
specify units explicitly, using `/second', `/minute', `/hour' or
`/day', or parts of them (so `5/second' is the same as `5/s').

--limit-burst

followed by a number, indicating the
maximum burst before the above limit kicks in.

This match can often be used with the LOG target to do rate-limited
logging. To understand how it works, let's look at the following
rule, which logs packets with the default limit parameters:

# iptables -A FORWARD -m limit -j LOG

The first time this rule is reached, the packet will be logged; in
fact, since the default burst is 5, the first five packets will be
logged. After this, it will be twenty minutes before a packet will be
logged from this rule, regardless of how many packets reach it. Also,
every twenty minutes which passes without matching a packet, one of
the burst will be regained; if no packets hit the rule for 100
minutes, the burst will be fully recharged; back where we started.

Note: you cannot currently create a rule with a recharge time
greater than about 59 hours, so if you set an average rate of one per
day, then your burst rate must be less than 3.

You can also use this module to avoid various denial of service
attacks (DoS) with a faster rate to increase responsiveness.

You can see that the first five packets are allowed to exceed the one
packet per second, then the limiting kicks in. If there is a pause,
another burst is allowed but not past the maximum rate set by the
rule (1 packet per second after the burst is used).

owner

This module attempts to match various characteristics of the packet
creator, for locally-generated packets. It is only valid in the
OUTPUT chain, and even then some packets (such as ICMP ping responses)
may have no owner, and hence never match.

--uid-owner userid

Matches if the packet was created by a process with the given
effective (numerical) user id.

--gid-owner groupid

Matches if the packet was created by a process with the given
effective (numerical) group id.

--pid-owner processid

Matches if the packet was created by a process with the given
process id.

--sid-owner sessionid

Matches if the packet was created by a process in the given session
group.

unclean

This experimental module must be explicitly
specified with `-m unclean or `--match unclean'. It does various
random sanity checks on packets. This module has not been audited,
and should not be used as a security device (it probably makes things
worse, since it may well have bugs itself). It provides no options.

The State Match

The most useful match criterion is supplied by the `state'
extension, which interprets the connection-tracking analysis of the
`ip_conntrack' module. This is highly recommended.

Specifying `-m state' allows an additional `--state' option, which
is a comma-separated list of states to match (the `!' flag indicates
not to match those states). These states are:

NEW

A packet which creates a new connection.

ESTABLISHED

A packet which belongs to an existing
connection (i.e., a reply packet, or outgoing packet on a connection
which has seen replies).

RELATED

A packet which is related to, but not part of, an
existing connection, such as an ICMP error, or (with the FTP module
inserted), a packet establishing an ftp data connection.

INVALID

A packet which could not be identified for some
reason: this includes running out of memory and ICMP errors which
don't correspond to any known connection. Generally these packets
should be dropped.

Now we know what examinations we can do on a packet, we need a way
of saying what to do to the packets which match our tests. This is
called a rule's target.

There are two very simple built-in targets: DROP and ACCEPT. We've
already met them. If a rule matches a packet and its target is one of
these two, no further rules are consulted: the packet's fate has been
decided.

There are two types of targets other than the built-in ones:
extensions and user-defined chains.

User-defined chains

One powerful feature which iptables inherits from
ipchains is the ability for the user to create new chains, in
addition to the three built-in ones (INPUT, FORWARD and OUTPUT). By
convention, user-defined chains are lower-case to distinguish them
(we'll describe how to create new user-defined chains below in
Operations on an Entire Chain).

When a packet matches a rule whose target is a user-defined chain, the
packet begins traversing the rules in that user-defined chain. If
that chain doesn't decide the fate of the packet, then once traversal
on that chain has finished, traversal resumes on the next rule in the
current chain.

Time for more ASCII art. Consider two (silly) chains: INPUT (the
built-in chain) and test (a user-defined chain).

Consider a TCP packet coming from 192.168.1.1, going to 1.2.3.4. It
enters the INPUT chain, and gets tested against Rule1 - no match.
Rule2 matches, and its target is test, so the next rule examined
is the start of test. Rule1 in test matches, but doesn't
specify a target, so the next rule is examined, Rule2. This doesn't
match, so we have reached the end of the chain. We return to the
INPUT chain, where we had just examined Rule2, so we now examine
Rule3, which doesn't match either.

User-defined chains can jump to other user-defined chains (but
don't make loops: your packets will be dropped if they're found to
be in a loop).

Extensions to iptables: New Targets

The other type of extension is a target. A target extension
consists of a kernel module, and an optional extension to
iptables to provide new command line options. There are
several extensions in the default netfilter distribution:

LOG

This module provides kernel logging of matching
packets. It provides these additional options:

--log-level

Followed by a level number or name. Valid
names are (case-insensitive) `debug', `info', `notice', `warning',
`err', `crit', `alert' and `emerg', corresponding to numbers 7
through 0. See the man page for syslog.conf for an explanation of
these levels. The default is `warning'.

--log-prefix

Followed by a string of up to 29 characters,
this message is sent at the start of the log message, to allow it to
be uniquely identified.

This module is most useful after a limit match, so you don't flood
your logs.

REJECT

This module has the same effect as `DROP', except
that the sender is sent an ICMP `port unreachable' error message.
Note that the ICMP error message is not sent if (see RFC 1122):

The packet being filtered was an ICMP error message in the
first place, or some unknown ICMP type.

The packet being filtered was a non-head fragment.

We've sent too many ICMP error messages to that destination
recently (see /proc/sys/net/ipv4/icmp_ratelimit).

REJECT also takes a `--reject-with' optional argument which alters the
reply packet used: see the manual page.

Special Built-In Targets

There are two special built-in targets: RETURN and
QUEUE.

RETURN has the same effect of falling off the end of a
chain: for a rule in a built-in chain, the policy of the chain is
executed. For a rule in a user-defined chain, the traversal continues
at the previous chain, just after the rule which jumped to this chain.

QUEUE is a special target, which queues the packet for
userspace processing. For this to be useful, two further components are
required:

a "queue handler", which deals with the actual mechanics of
passing packets between the kernel and userspace; and

a userspace application to receive, possibly manipulate, and
issue verdicts on packets.

The standard queue handler for IPv4 iptables is the ip_queue module,
which is distributed with the kernel and marked as experimental.

The following is a quick example of how to use iptables to queue packets
for userspace processing:

With this rule, locally generated outgoing ICMP packets (as created with,
say, ping) are passed to the ip_queue module, which then attempts to deliver
the packets to a userspace application. If no userspace application is
waiting, the packets are dropped.

To write a userspace application, use the libipq API. This is
distributed with iptables. Example code may be found in the testsuite
tools (e.g. redirect.c) in CVS.

The status of ip_queue may be checked via:

/proc/net/ip_queue

The maximum length of the queue (i.e. the number packets delivered
to userspace with no verdict issued back) may be controlled via:

/proc/sys/net/ipv4/ip_queue_maxlen

The default value for the maximum queue length is 1024. Once this limit
is reached, new packets will be dropped until the length of the queue falls
below the limit again. Nice protocols such as TCP interpret dropped packets
as congestion, and will hopefully back off when the queue fills up. However,
it may take some experimenting to determine an ideal maximum queue length
for a given situation if the default value is too small.

A very useful feature of iptables is the ability to group
related rules into chains. You can call the chains whatever you want,
but I recommend using lower-case letters to avoid confusion with the
built-in chains and targets. Chain names can be up to 31 letters
long.

Creating a New Chain

Let's create a new chain. Because I am such an imaginative fellow,
I'll call it test. We use the `-N' or `--new-chain' options:

# iptables -N test
#

It's that simple. Now you can put rules in it as detailed above.

Deleting a Chain

Deleting a chain is simple as well, using the `-X' or `--delete-chain'
options. Why `-X'? Well, all the good letters were taken.

# iptables -X test
#

There are a couple of restrictions to deleting chains: they must be
empty (see
Flushing a Chain below) and they
must not be the target of any rule. You can't delete any of the three
built-in chains.

If you don't specify a chain, then all user-defined chains
will be deleted, if possible.

Flushing a Chain

There is a simple way of emptying all rules out of a chain, using the
`-F' (or `--flush') commands.

# iptables -F FORWARD
#

If you don't specify a chain, then all chains will be flushed.

Listing a Chain

You can list all the rules in a chain by using the `-L' (or `--list')
command.

The `refcnt' listed for each user-defined chain is the number of rules
which have that chain as their target. This must be zero (and the
chain be empty) before this chain can be deleted.

If the chain name is omitted, all chains are listed, even empty ones.

There are three options which can accompany `-L'. The `-n' (numeric)
option is very useful as it prevents iptables from trying to
lookup the IP addresses, which (if you are using DNS like most people)
will cause large delays if your DNS is not set up properly, or you
have filtered out DNS requests. It also causes TCP and UDP ports to
be printed out as numbers rather than names.

The `-v' options shows you all the details of the rules, such as the
the packet and byte counters, the TOS comparisons, and the interfaces.
Otherwise these values are omitted.

Note that the packet and byte counters are printed out using the
suffixes `K', `M' or `G' for 1000, 1,000,000 and 1,000,000,000
respectively. Using the `-x' (expand numbers) flag as well prints the
full numbers, no matter how large they are.

Resetting (Zeroing) Counters

It is useful to be able to reset the counters. This can be done with
the `-Z' (or `--zero') option.

Consider the following:

# iptables -L FORWARD
# iptables -Z FORWARD
#

In the above example, some packets could pass through between the `-L'
and `-Z' commands. For this reason, you can use the `-L' and `-Z'
together, to reset the counters while reading them.

Setting Policy

We glossed over what happens when a packet hits the end of a built-in
chain when we discussed how a packet walks through chains earlier. In
this case, the policy of the chain determines the fate of the
packet. Only built-in chains (INPUT, OUTPUT and
FORWARD) have policies, because if a packet falls off the end
of a user-defined chain, traversal resumes at the previous chain.